The present invention is directed to an improved flow rate sensor. In particular, the flow rate sensor disclosed is optimized for use with high flow rate liquid applications, such as a household appliance like a washing machine or dishwasher, which typically consume water at a rate of several gallons per minute. The sensor could be used with any other application where it is desirable to quickly and reliably measure flow rate.
Recently, a great deal of concern has developed regarding environmentally-friendly appliances. Such appliances use less electricity and only the minimum necessary amount of water to accomplish the designated task, such as washing clothes. An example of this concern is demonstrated by the EPA's "Energy Star" program, which sets requirements for power consumption in the home electronics area and designates certain appliances as "Energy Star" compliant when they meet those requirements.
The present invention is also aimed at making appliances more environmentally friendly, but in the area of water consumption. By integrating the flow rate sensor described in this application with a washing machine (or other appliance) having a minimum level of intelligence, a "smart" appliance is possible. Such a "smart" machine includes some type of computer controller, such as a microprocessor or microcontroller ("MCU"), which is coupled to a variety of sensors. The flow rate sensor of the present invention could be one of those sensors. Other sensors may include temperature sensors, level sensors, humidity sensors, optical sensors, etc. The flow rate sensor of the present invention can be used alone, or in combination with other sensors, to provide the "smart" appliance with precise information regarding the amount of water being consumed. In this manner, and under computer-implemented software instructions operating at the MCU, the water consumption of the appliance can be made optimally efficient depending upon the size of the load, water temperature, and any other conditions.
Presently known flow rate sensors include: (1) "turbine-type", including paddle wheels or hinged vanes; (2) pressure gradient sensors; (3) vortex-type sensors; (4) ultra-sonic sensors; and (5) thermal transport sensors. Sensor types (1) through (4) suffer from several disadvantages that make them undesirable for use with household appliances that consume a large amount of water, such as: (i) low accuracy over a wide range of flow rates and inlet temperatures; (ii) low reliability due to moving parts; (iii) more complex signal conditioning and power circuitry; (iv) large size; (v) expensive; and (vi) slow and unstable readings at high flow rates, to name but a few.
Another type of presently known flow rate sensor uses a concept known as "thermal transport." Known thermal transport sensors include at least two temperature measuring devices located at two positions within the sensor, an upstream and a downstream temperature measuring device. The upstream temperature measuring device provides a reading of ambient flow temperature. The downstream device is in close proximity to a heating element that maintains the temperature of the downstream temperature measuring device at the measured ambient level. As liquid flows past the downstream temperature measuring device it is cooled, and thus more energy must be applied to the heating element to maintain it at the ambient level. The flow rate of the liquid is calculated from a measurement of the energy input to the heating element in order to maintain the downstream temperature measuring device at the ambient temperature level. Examples of this type of thermal transport sensor are set forth in U.S. Pat. Nos. 4,480,467 and 4,384,578.
This type of thermal transport sensor suffers from several disadvantages, including: (1) relatively slow response time and unstable readings, particularly for high flow rate applications; (2) more complex electronic circuitry is required to monitor energy consumption; (3) cost; (4) complexity; (5) low sensitivity; (6) inability to operate over a wide range of flow temperatures; and (7) erratic readings due to turbulent flows, to name a few.
Therefore, there remains a need in this art for a low cost flow rate sensor that provide fast response times and stable readings under a variety of conditions, including a wide range of flow temperatures and flow velocities.
There remains a more particular need for such a sensor that provides fasts stable readings at relatively high flow rates, such as are associated with household appliances.
There remains still a more particular need for such a sensor that can be used in conjunction with a household appliance, such as a washing machine or dishwasher, to provide an environmentally optimized "smart" appliance.